Chemically strengthened glass, substrate for information recording medium and information recording medium

ABSTRACT

A glass for use in chemical reinforcement for use in a substrate of an information recording medium, having a composition comprising, denoted as mol %:  
                                           SiO 2     47 to 70%          Al 2 O 3     1 to 10%                   (where the total of SiO 2  and Al 2 O 3  is 57 to 80%)                       CaO   2 to 25%         BaO   1 to 15%         Na 2 O   1 to 10%         K 2 O   0 to 15%                   (where the total of Na 2 O and K 2 O is 3 to 16%)                       ZrO 2     1 to 12%         MgO   0 to 10%         SrO   0 to 15%                   (where the ratio of the content of CaO to the total of MgO, CaO,         SrO, and BaO is greater than or equal to 0.5)                       ZnO   0 to 10%                   (where the total of MgO, CaO, SrO, BaO, and ZnO is 3 to 30%)                       TiO 2     0 to 10%                   and the total content of the above-stated components is greater         than or equal to 95%.                                                                                            
 
     A glass for use in chemical reinforcement for use in the substrate of an information recording medium employed in a perpendicular magnetic recording system, in which the glass exhibits the glass transition temperature is greater than or equal to 600° C. A substrate for use in an information recording medium consisting of the above glass and being chemically reinforced. A substrate for an information recording medium consisting of a chemically reinforced glass having a glass transition temperature of greater than or equal to 600° C. and exhibiting a bending strength following heating for two hours at 570° C. of greater than or equal to 15 kgf/mm 2 . An information recording medium comprising an information recording layer on the above substrate for an information recording medium. The present invention provides glass having both high thermal resistance and high strength, a substrate for use in information recording media comprised of this glass, and an information recording medium employing such a substrate.

TECHNICAL FIELD

The present invention relates to chemically reinforced glass having highthermal resistance and high strength. The present invention furtherrelates to a substrate for an information recording medium of chemicallyreinforced glass having both high thermal resistance and high strength,and to an information recording medium employing this substrate. Inparticular, the present invention relates to a substrate for a magneticrecording medium suited to the manufacturing with a high-temperaturesputtering device of magnetic films employed in perpendicular magneticrecording systems, and to a magnetic recording medium.

BACKGROUND ART

In recent years, as recording has been conducted at ever greater densityin information recording devices such as magnetic disks, typified byhard disks, there has been demand for a change from longitudinalmagnetic recording systems to perpendicular magnetic recording systems.That is, it has been pointed out that in longitudinal magnetic recordingsystems, since the magnetic zone is readily rotated by heat at aboutroom temperature, it becomes impossible to write and data that have beenwritten tend to be lost as the recording density increases. Thisphenomenon is known as the problem of thermal fluctuation and isbecoming an ever greater impediment in longitudinal magnetic recordingmethods. Accordingly, in response to the problem of thermal fluctuationin longitudinal magnetic recording methods, there has been an activeresearch of the practical use of perpendicular magnetic recordingmethods in recent years.

Known film structures employed in perpendicular magnetic recordingmethods include a single-layer film formed over a perpendicular magneticrecording layer on a nonmagnetic substrate, a double-layer film obtainedby sequentially stacking a soft magnetic layer and a magnetic recordinglayer on a nonmagnetic substrate, and a three-layer film obtained bysequentially stacking a hard magnetic layer, a soft magnetic layer, anda magnetic recording layer on a nonmagnetic substrate. Of these, thetwo-layer film and three-layer film are better suited to achieving highrecording densities and the maintenance of a stable magnetic moment thanthe single-layer film, leading to substantial development focusing onpractical use in recent years. The improvement of the characteristics ofsuch multilayer film perpendicular magnetic recording media requiresfilm formation with a high-temperature film forming unit such as ahigh-temperature sputtering device and high-temperature treatmentfollowing film formation.

When employing a glass substrate affording good processing propertiesand reliability as the substrate of various information recording media,including the magnetic recording media employed in the above-mentionedperpendicular magnetic recording method, it is necessary to solveproblems such as the following.

The glass substrate for information recording media is subjected toprecision polishing such as lapping and polishing to impart extremelyhigh flatness and surface smoothness. However, there is a problem inthat since the substrate is exposed to elevated temperatures in thecourse of forming a film serving as the information recording layer, theglass softens and deforms unless the thermal resistance of the substrateis high, precluding use as an information recording medium. Thus, thereis a need for a glass material with high thermal resistance that doesnot deform when exposed to elevated temperatures.

This high thermal resistance is an important characteristic for ensuringflatness in a static state. Flatness is also demanded in high-speedrotation since the reading and writing of information is conducted withthe information recording medium being rotated at high speed. Thus, asubstrate that does not yield at high-speed rotation, that is, asubstrate of a material with a high Young's modulus, is required.

In the above-mentioned film-forming step, the glass substrate is heldand conveyed into and out of a high-temperature film-forming device. Inthe course of this conveyance, a substrate that has been heated to anelevated temperature is rapidly cooled, subjecting the glass substrate,particular the portion being held, to substantial stress due to thermalcontraction. Similarly, there is also a problem in that the glasssubstrate is also sometimes subjected to substantial thermal shockduring high-temperature heat processing following film formation, withthe substrate being damaged by this shock. Thus, there is a need for ahigh-strength glass substrate capable of adequately withstanding thermalshock.

Further, an information recording medium such as that mentioned aboverotates at an extremely high speed of several thousand rpm duringoperation. Thus, there is a strong need to increasing the strength ofthe glass substrate to prevent damage during high-speed rotation.

An example of a glass employable as substrate in an informationrecording medium is the chemically reinforceable alumina silicate glassdisclosed in Patent Reference 1 (Japanese Unexamined Patent Publication(KOKAI) Heisei No. 10-72238). However, from 8.5 to 15.5 mol % of Na₂O isincorporated into the glass described in Patent Reference 1 to enhancethe glass melt property and increase ion-exchange efficiency forchemical reinforcement. Na₂O has the effect of decreasing the Young'smodulus of the glass. Thus, the glass described in Patent Reference 1has a low Young's modulus and substrates produced from the glass havepoor flatness when rotated at high speed. Nor is application toinformation recording media employed in perpendicular magnetic recordingsystems suggested in any way in Patent Reference 1.

Accordingly, the present invention has for its object to provide glasshaving both high thermal resistance and high strength, a substrate foruse in information recording media comprised of this glass, and aninformation recording medium employing such a substrate.

DISCLOSURE OF THE INVENTION

The means of achieving the above-stated object of the present inventionare as follows:

(1) A glass for use in chemical reinforcement for use in the substrateof an information recording medium, having a composition comprising,denoted as mol %: SiO₂ 47 to 70%  Al₂O₃ 1 to 10% (where the total ofSiO₂ and Al₂O₃ is 57 to 80%) CaO 2 to 25% BaO 1 to 15% Na₂O 1 to 10% K₂O0 to 15% (where the total of Na₂O and K₂O is 3 to 16%) (where the ratioof the total of CaO and BaO to the total of MgO, CaO, SrO, and BaO isgreater than or equal to 0.5) ZrO₂ 1 to 12% MgO 0 to 10% SrO 0 to 15%ZnO 0 to 10% (where the total of MgO, CaO, SrO, BaO, and ZnO is 3 to30%) TiO₂ 0 to 10% and the total content of the above-stated componentsis greater than or equal to 95%.

-   (2) The glass for use in chemical reinforcement described in (1)    characterized in that the ratio of the BaO content to the total    content of MgO, CaO, SrO, and BaO is greater than or equal to 0.15.-   (3) A glass for use in chemical reinforcement for use in a substrate    of an information recording medium employed in a perpendicular    magnetic recording system, in which the glass exhibits the glass    transition temperature of greater than or equal to 600° C.-   (4) The glass for use in chemical reinforcement described in any    of (1) to (3) above having a Young's modulus of greater than or    equal to 75 GPa.-   (5) A substrate for use in an information recording medium    characterized by consisting of any of the glasses described in (1)    to (4) above and being chemically reinforced.-   (6) The substrate for use in an information recording medium    described in (5) above employing a chemically reinforced glass in    which the bending strength following heating for two hours at    570° C. to is greater than or equal to 15 kgf/mm².-   (7) A substrate for an information recording medium characterized by    consisting of a chemically reinforced glass having a glass    transition temperature of greater than or equal to 600° C. and    exhibiting a bending strength following heating for two hours at    570° C. of greater than or equal to 15 kgf/mm².-   (8) The substrate for an information recording medium described in    any of (5) to (7) above in which, when the bending strength of the    glass constituting the substrate prior to chemical reinforcement is    denoted as f_(b) and the bending strength of the glass when    maintained for two hours at a temperature T [° C.] (where T denotes    any temperature of from 20 to 570° C.) after having been chemically    reinforced is denoted as f_(T), the value of (f_(T)−f_(b))/f_(b) is    greater than or equal to 0.5.-   (9) The substrate for use in an information recording medium of (7)    above wherein the value of (f₂₀−f_(b))/f_(b) for the bending    strength f₂₀ at T=20° C. is greater than or equal to 1.-   (10) The substrate for use in an information recording medium    described in any of (5) to-   (9) above wherein the average coefficient of linear expansion at 30    to 300° C. of the glass constituting the substrate is greater than    or equal to 60×10⁻⁷K⁻¹.-   (11) The substrate for use in an information recording medium    described in any of (5) to-   (10) above that is chemically reinforced by an ion exchange    treatment in which sodium ions are replaced with potassium ions.-   (12) The substrate for use in an information recording medium    described in any of (5) to (11) above that is employed as a    substrate for an information recording medium employed in a    perpendicular magnetic recording system.-   (13) An information recording medium characterized by comprising an    information recording layer on the substrate for an information    recording medium described in any of (5) to (11) above.-   (14) The information recording medium described in (13) above in the    form of a magnetic recording medium employed in a perpendicular    magnetic recording system.-   (15) The information recording medium described in (13) or (14)    above characterized by being manufactured by subjecting a substrate    having an information recording layer to a heat treatment at a    maximum temperature of 300 to 600° C.-   (16) A method of manufacturing an information recording medium    comprising a step of forming a multilayered film comprising an    information recording layer on the substrate for an information    recording medium described in any of (5) to (11) above,    characterized by further comprising the heating of the substrate on    which the multilayered film has been formed to a temperature of from    300 to 600° C.

The substrate for an information recording medium of the presentinvention is of high strength due to chemical reinforcement; has a glasstransition temperature of greater than or equal to 600° C., desirablygreater than or equal to 620° C.; does not deform when exposed totemperatures of about 300 to 600° C., preferably about 400 to 600° C.,is capable of maintaining a good shape; and has a high Young's modulus,thus affording good stability at high-speed rotation, deforming littleeven when rapidly rotated. Further, since the substrate for aninformation recording medium of the present invention has a value(f_(T)−f_(b))/f_(b) of greater than or equal to 0.5, it is able tomaintain adequate bending strength even when subjected to a heattreatment of about 300 to 600° C., desirably about 400 to 600° C. Stillfurther, since the substrate for an information recording medium of thepresent invention has a thermal expansion characteristic close to thatof a metal, glass substrates for information processing media that canbe firmly secured with metal fixtures can be provided. Further, sincethe information recording medium of the present invention has aninformation recording layer on such an information recording medium,high-temperature treatment can be conducted to impart high strength andhigh rotational stability. The information recording medium of thepresent invention is particularly suited to magnetic recording mediaemployed in perpendicular magnetic recording systems, thus making itpossible to provide information recording media of correspondinglyhigher recording density.

BEST MODE OF IMPLEMENTING THE INVENTION

The present invention is described in greater detail below.

The first glass for use in chemical reinforcement (“glass I”hereinafter) of the present invention is a chemically reinforced glassfor use in the substrate of an information recording medium, having acomposition, expressed in mol %, comprising: SiO₂ 47 to 70%  Al₂O₃ 1 to10% (where the total of SiO₂ and Al₂O₃ is 57 to 80%) CaO 2 to 25% BaO 1to 15% Na₂O 1 to 10% K₂O 0 to 15% (where the total of Na₂O and K₂O is 3to 16%) ZrO₂ 1 to 12% MgO 0 to 10% SrO 0 to 15% ZnO 0 to 10% (where thetotal of MgO, CaO, SrO, BaO, and ZnO is 3 to 30%) TiO₂ 0 to 10% wherethe total content of the above-stated components is greater than orequal to 95%.In the above-described glass, the ratio of CaO to the total content ofMgO, CaO, SrO, and BaO (CaO/(MgO+CaO+SrO+BaO)) is desirably greater thanor equal to 0.5, preferably greater than or equal to 0.55, and morepreferably greater than or equal to 0.6. The role and composition rangeof each component of glass I will be described here.

SiO₂ is a major component of the network former of the glass. When thecontent thereof is less than 47%, the thermal stability of the glasstends to decrease and the glass tends to devitrify. Durability alsodecreases sharply and extreme corrosion tends to occur due to thecleaning solution of hydrofluorosilicic acid employed to clean the glasssurface. When 70% is exceeded, the Young's modulus of the glassdecreases and viscosity at high temperature increases, causing glassmelt properties to deteriorate sharply. Accordingly, the SiO₂ content ofglass I falls within the range of 47 to 70%, preferably 50 to 67%.

Al₂O₃ is a component that both contributes greatly to improving thedurability and thermal resistance of the glass and is extremelyimportant as a component enhancing the stability and rigidity of theglass structure. However, at a content of less than 1%, the effect ofinhibiting the dissolution of alkali out of the glass tends to diminish,making it difficult to obtain a glass with good durability. When 10% isexceeded, the high-temperature melt property of the glass deteriorates.Accordingly, the content range is set to from 1 to 10%, preferably from2 to 10%.

In the glass for use in chemical reinforcement of the present invention,the total content of SiO₂ and Al₂O₃ falls within a range of 57 to 80%,preferably 57 to 79%. When this total content is less than 57%, there isa risk of the glass being inadequately durable. When 80% is exceeded,the Young's modulus and the coefficient of thermal expansion decrease.Further, viscosity at high temperature increases, compromising meltproperties.

MgO, CaO, SrO, BaO, and ZnO are components that decrease the viscosityof the glass melt, promote melting, increase the Young's modulus, andincrease the coefficient of thermal expansion. However, when the totalcontent thereof exceeds 30%, the durability of the glass tends todeteriorate, heat stability decreases, and devitrification tends tooccur. Further, when the total content is less than 3%, the glasstransition temperature tends to decrease and high-temperature viscosityincreases. Further, when an alkali metal oxide is introduced instead ofan alkaline earth metal oxide, the Young's modulus drops. Accordingly,the total content of MgO, CaO, SrO, BaO, and ZnO in glass I is set tofrom 3 to 30%, preferably from 3 to 25%.

CaO is an important component for increasing the Young's modulus andcoefficient of thermal expansion, as well as decreasing melt viscosity.However, when the quantity of CaO incorporated is less than 2%, theeffect achieved is slight. When 25% is exceeded, stability tends todeteriorate. Thus, the content range is set to from 2 to 25%, preferablyfrom 3 to 20%.

BaO is incorporated in a quantity of not less than 1% into the glass forchemical reinforcement of the present invention to help increase thecoefficient of thermal expansion and enhance durability. However, whenmore than 15% is introduced, durability tends to deteriorate.Introducing BaO greatly increases the specific gravity of the glass.Accordingly, the content of BaO in glass I is set to within the range offrom 1 to 15%, preferably from 1 to 14%.

Adding MgO, ZnO, and SrO in such a manner that the total content of MgO,CaO, SrO, BaO, and ZnO does not exceed the above stated range stabilizesthe structure of the glass, raises the Young's modulus, and increasesthe coefficient of thermal expansion. A better effect is achieved byincorporating small amounts of various bivalent components than byincorporating a large quantity of one from among MgO, ZnO, and SrO.Thus, the contents are set to: 0 to 10% MgO, 0 to 15% SrO, and 0 to 10%ZnO; desirably 0 to 10% SrO, 0 to 8% ZnO, and 0 to 5% MgO; andpreferably 0 to 1% MgO, 0 to 1% SrO, and 0 to 1% ZnO.

The amount of alkali metal oxide incorporated is desirably kept below aprescribed amount to prevent a drop in the glass transition temperature.However, when the amount of alkali metal oxide incorporated is keptdown, glass melt properties deteriorate or the coefficient of thermalexpansion tends to drop below the optimal range for a substrate employedin an information recording medium. Accordingly, to prevent such a dropin melt properties and such a reduction in the coefficient of thermalexpansion, an alkaline earth metal oxide is introduced in the presentinvention. Since CaO is an alkaline earth metal oxide of relatively lowmolecular weight, it affords the advantage of tending not to increasethe specific gravity of the glass. Although MgO also has the effect ofinhibiting an increase in specific gravity, it has a greater tendency toreduce the chemical reinforcement effect than CaO. Thus, the proportionof CaO in the alkaline earth metal oxide is desirably high.Specifically, the quantities of each of the above-stated components aredesirably adjusted so that the ratio of CaO/(MgO+CaO+SrO+BaO) is greaterthan or equal to 0.5, desirably greater than or equal to 0.55, andpreferably greater than or equal to 0.6. Further, since alkaline earthmetal oxides increase the melt properties of the glass withoutcompromising the glass transition temperature as set forth above,increasing the coefficient of thermal expansion, the total content ofMgO, CaO, SrO, and BaO is desirably from 10 to 30%, preferably from 12to 30%, and more preferably from 12 to 25%.

Further, among the alkaline earth metal oxides, BaO enhances thedevitrification property of the glass, and compared to MgO, CaO, andSrO, plays a greater role in increasing the coefficient of thermalexpansion. Accordingly, the ratio of the content of BaO relative to thetotal content of MgO, CaO, SrO, and BaO (BaO/(MgO+CaO+SrO+BaO)incorporated is greater than or equal to 0.15, desirably greater than orequal to 0.16, and preferably greater than or equal to 0.17.

Na₂O and K₂O are components that are useful in decreasing the viscosityof the glass melt, promoting melting of the glass, and greatlyincreasing thermal expansion. In particular, Na₂O is used to effectreinforcement by substituting by ion exchange potassium ions in themolten salt for sodium ions present in the glass. However, when thetotal content of Na₂O and K₂O exceeds 15%, not only does chemicaldurability deteriorate, but a large amount of alkali precipitates out onthe surface of the glass, creating the risk of corrosion of aninformation recording layer, such as a magnetic layer. There are alsocases in which the glass transition temperature drops, precluding thenecessary thermal resistance. By contrast, when the total content isless than 3%, good chemical reinforcement becomes difficult, the glassmelt properties deteriorate, and it becomes difficult to achieveprescribed thermal expansion characteristics. Accordingly, the totalcontent of Na₂O and K₂O in glass I is 3 to 16%, desirably 3 to 15%,preferably 4 to 14%, and more preferably, 4 to 12%.

Na₂O is an important component for achieving chemical reinforcementwithout decreasing the glass transition temperature. A content ofgreater than or equal to one % achieves good chemical reinforcement.Although to a lesser degree than K₂O, Na₂O has the effect of increasingthe coefficient of thermal expansion. Since a large amount of the Na₂Oprecipitates out on the glass surface, the upper content limit is set to8%. Accordingly, in glass I, the content of Na₂O is from 1 to 10%,desirably from 1 to 9%, preferably from 1 to 8%, more preferably from 1to 7%, and even more preferably from 1 to 5%.

K₂O is an important component that greatly increases the coefficient ofthermal expansion and reduces the ratio of precipitation onto thesurface of the glass. The K₂O content desirably exceeds 0%, preferablyexceeds 1%, more preferably exceeds 2%, and even more preferably exceeds4% to contribute to achieving desired thermal expansion characteristicsand glass melt properties while suppressing the amount of alkalidissolving out. However, when the content exceeds 15%, the durability ofthe glass decreases and thermal resistance deteriorates due to a drop inthe glass transition temperature. Accordingly, the content of K₂O inglass I falls within a range of from 0 to 15%, desirably a range of fromgreater than 0% to less than or equal to 15%, preferably a range of from1 to 15%, more preferably a range of from 2 to 15%, and even moredesirably a range of from 4 to 13%.

ZrO₂ and TiO₂ are components that increase the chemical durability andrigidity of the glass. The addition of small quantities of ZrO₂ and TiO₂improves the durability, modulus of elasticity, and brittleness of theglass. However, the introduction of ZrO₂ and TiO₂ also sharply increasesspecific gravity. When large quantities are introduced, there is aproblem in that the glass has a strong tendency to devitrify.

Further, ZrO₂ is a component the introduction of which raises theYoung's modulus. The introduction of one % or more achieves theabove-stated effects, but when more than 12% is incorporated, thespecific gravity increases. Accordingly, in glass I, the content of ZrO₂is set to from 1 to 12%, desirably from 1 to 10%, and preferably from3to 10%.

TiO₂ has a lesser tendency than ZrO₂ to increase the Young's modulus,but produces little increase in specific gravity. When TiO₂ is added ina quantity exceeding 10%, specific gravity increases and the glassdevitrifies. Accordingly, in glass I, the content of TiO₂ is set to from0 to 10%, desirably from 0 to 8%. In consideration of water repellence,the content of TiO₂ is desirably 0%.

To achieve the above-stated desired objects, the total content of theabove-listed components (SiO₂, Al₂O₃, CaO, BaO, Na₂O, K₂O, ZrO₂, MgO,SrO, ZnO, and TiO₂) is greater than or equal to 95%, desirably greaterthan or equal to 97%, and preferably greater than or equal to 98%.However, when necessary, the following components may also be added.

Li₂O may be incorporated into glass I in addition to the abovecomponents. Li₂O has the effect of increasing thermal expansion andraising the Young's modulus, but the proportion precipitating out ontothe glass surface is high, and the addition of even a small quantitygreatly lowers the glass transition temperature. Accordingly, thequantity incorporated is desirably kept to 3% or less, preferably 1% orless. More preferably, none is introduced. When Li₂O is introduced,chemical reinforcement can be conducted by immersion in a molten saltcontaining potassium ions. Through the exchange of Li ions and Na ions,greater mechanical strength can be achieved. Thus, immersion in a moltensalt containing sodium ions and potassium ions (for example, a mixedmolten salt of sodium nitrate and potassium nitrate) is desirablyemployed for chemical reinforcement.

Rare earth elements can be introduced into glass I as optionalcomponents. Rare earth elements function to increase the thermalresistance, durability, and modulus of elasticity of a glass substrate,but they are expensive. Accordingly, from the perspective of cost, it isdesirable not to incorporate rare earth elements. That is, the desiredobject can be achieved in glass I without incorporating rare earthelements.

From the perspective of achieving a better Young's modulus, thermalresistance, and durability, the incorporation of rare earth elements isdesirable. When incorporating rare earth elements, they are desirablyincorporated in a ratio of less than or equal to 5%, preferably lessthan or equal to 3%, based on the oxide.

Examples of the above-mentioned rare earth elements are Y, La, Gd, Yb,Pr, Sc, Sm, Tb, Dy, Nd, Eu, Ho, Er, Tm, and Lu. Examples in the form ofoxides are Y₂O₃, La₂O₃, Gd₂O₃, Yb₂O₃, Pr₂O₃, Sc₂O₃, Sm₂O₃, Tb₂O₃, Dy₂O₃,Nd₂O₃, Eu₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, and Lu₂O₃. Y₂O₃ is desirably employedas the oxide of a rare earth element. When employing Y₂O₃, there is asubstantial increase in the Young's modulus without a large increase inspecific gravity. However, there is a sharp drop in the stability of theglass. Thus, the content is desirably less than or equal to 8%,preferably less than or equal to 5%. Whether or not to incorporate arare earth element may be suitably determined based on the above-statedcircumstances.

In addition to the above-listed components, a defoaming agent may beincorporated into glass I to improve the melt properties, clarity, andmoldability of the glass. Examples are As₂O₃, Sb₂O₃, fluorides,chlorides, and SO₃. The quantity incorporated need only fall within asuitable range for the defoaming agent employed; an overall ratio ofless than or equal to 2 weight % may serve as an outer percentage. Inparticular, Sb₂O₃ and As₂O₃ are antifoaming agents with strongantifoaming effects, keeping foaming in the glass to an extremely lowlevel or preventing it entirely. When residual bubbles in the glassappear on the substrate surface during polishing, they producedistortion that compromises the smoothness of the surface. Of these, theincorporation of Sb₂O₃ is preferable. In consideration of the effect onthe environment, it is desirable not to employ arsenic compounds such asAs₂O₃. In the present invention, the introduction of a foaming agent inthe form of from 0 to 1 weight % of just Sb₂O₃, preferably from 0.1 to 1weight % of just Sb₂O₃ as an outer percentage, is desirable.

It is possible to combine the various components within the above-stateddesirable composition ranges into even more desirable compositionranges. Among these, one such set of desirable composition ranges is:SiO₂ 50 to 67%  Al₂O₃ 2 to 10% (where the total of SiO₂ and Al₂O₃ is 57to 79%) CaO 3 to 20% BaO 1 to 14% MgO 0 to 10% SrO 0 to 10% ZnO 0 to 8% (where the total of MgO, CaO, SrO, BaO and ZnO is 4 to 30%) Na₂O 1 to10% K₂O greater than 0%, less than or equal to 13% (where the totalquantity of Na₂O and K₂O is from 5 to 14%) ZrO₂ 1 to 10% and TiO₂ 0 to8%. 

In the above more desirable composition ranges, the total quantity ofMgO, CaO, SrO, BaO, and ZnO is from 3 to 25%. In the above composition,the content of Na₂O is desirably from 1 to 9%, preferably from 1 to 5%.The content of ZrO₂ is preferably from 3 to 10%.

In a mode not employing rare earth elements, the total content of SiO₂,Al₂O₃, CaO, BaO, MgO, SrO, ZnO, Na₂O, K₂O, ZrO₂, and TiO₂ is desirably100%. The above-described defoaming agent is also desirably added tothis composition. The alkali metal oxides are desirably limited to Na₂Oand K₂O from the perspective of enhancing melt properties and thermalresistance while suppressing the dissolution of alkali. In morepreferable compositions, the total quantities of SiO₂, Al₂O₃, CaO, BaO,Na₂O, K₂O, and ZrO₂ amount to 100%; the total quantities of SiO₂, Al₂O₃,CaO, BaO, MgO, Na₂O, K₂O, and ZrO₂ amount to 100%; and the totalquantities of SiO₂, Al₂O₃, CaO, BaO, Na₂O, K₂O, ZrO₂, and TiO₂ amount to100%. The above-described foaming agent may be added within theabove-stated compositional ranges. The addition of Sb₂O₃ as foamingagent is particularly desirable. The quantity incorporated desirablyfalls within a range of from 0 to 1 weight %, preferably 0.1 to 1 weight%, as an outer percentage.

In a mode incorporating alkaline earth elements, the total quantities ofSiO₂, Al₂O₃, CaO, BaO, MgO, SrO, ZnO, Li₂O, Na₂O, K₂O, ZrO₂, TiO₂, B₂O₃,and rare earth element oxides desirably amount to 100%. Therein, it isdesirable to keep the total quantity of rare earth element oxides to 5%or less. As defoaming agents, As₂O₃, Sb₂O₃, fluorides, chlorides, andSO₃ may be added in suitable quantities to the glass. However, thequantity added is desirably kept to 2% or less, preferably 1% or less,as an outer percentage. Due to environmental concerns, it is desirablenot to employ arsenic compounds such as As₂O₃. Further, the introductionof from 0.1 to 1 weight % of Sb₂O₃ as an outer percentage is desirableto reduce bubbles.

In all of the above-described glasses, glass melt properties are good,no undissolved material is observed in the glass, and no crystal grainsare observed in the glass. That is, each of the glasses is in anamorphous state.

The glass transition temperature of glass I is desirably greater than orequal to 600° C., preferably greater than or equal to 620° C., morepreferably greater than or equal to 650° C., and even more preferablygreater than or equal to 660° C.

The second glass for use in chemical reinforcement (“glass II”hereinafter) of the present invention exhibits a glass transitiontemperature of greater than or equal to 600° C., desirably greater thanor equal to 620° C., and can be employed in substrates for informationrecording media employed in perpendicular magnetic recording systems bybeing subjected to chemical reinforcement. Since it exhibits a glasstransition temperature of greater than or equal to 600° C., preferablygreater than or equal to 620° C., it does not deform when exposed tohigh temperatures of about 400-600° C., maintaining a good shape. Thus,glass II exhibits resistance to heat in the high-temperature filmformation and high-temperature heat treatment to which magneticrecording mediums employed in a perpendicular magnetic recording systemsare subjected. In particular, it is suited to use as a substrate for amagnetic recording medium employed in a perpendicular magnetic recordingsystem. The glass transition temperature of glass II is preferablygreater than or equal to 650° C., more preferably greater than or equalto 660° C. The composition of glass II comprises SiO₂, Al₂O₃, ZrO₂,Na₂O, and a total of 3 to 15 mol % of K₂O, and a total of 3 to 30 mol %of MgO, CaO, SrO, BaO, and ZnO (where the ratio of the content of CaO tothe combined content of MgO, CaO, SrO, and BaO is greater than or equalto 0.5). In these composition ranges, the ratio of the content of BaO tothe total content of MgO, CaO, SrO, and BaO, BaO/(MgO+CaO+SrO+BaO) whengiven as mol %, is greater than or equal to 0.15. Such glasses desirablycomprise 47-70 mol % of SiO₂, 1 to 10 mol % of Al₂O₃ (where the totalcontent of SiO₂ and Al₂O₃ is from 57 to 80 mol %), and 1 to 12 mol % ofZrO₂. Glasses incorporating SiO₂, Al₂O₃, CaO, BaO, Na₂O, K₂O, and ZrO₂,as well as glasses comprising SiO₂, Al₂O₃, CaO, BaO, MgO, Na₂O, K₂O, andZrO₂, are desirable. Both such glasses also desirably fall within thecompositional ranges of glass I.

Due to the above-stated compositions, glasses I and II exhibit a Young'smodulus of greater than or equal to 75 GPa, which is much higher thanthe Young's modulus (about 70 GPa) of the aluminum substratesconventionally employed in information recording media. Thus, it ispossible to provide a substrate for information recording media havinghigh strength, good stability during high speed rotation, and undergoinglittle deformation (elastic deformation) of the substrate when rotatedat high speed. The Young's modulus of glass I and glass II is preferablygreater than or equal to 78 GPa. To obtain glass with good stability,the Young's modulus is desirably kept to less than or equal to 100 GPa.The Young's modulus does not change as the result of chemicalreinforcement.

The first substrate for information recording media (“substrate A”hereinafter) of the present invention is characterized by beingcomprised of glass I or glass II and being chemically reinforced.

The second substrate for information recording media (“substrate B”hereinafter) of the present invention is characterized by beingcomprised of a chemically reinforced glass having a glass transitiontemperature of greater than or equal to 600° C., preferably greater thanor equal to 620° C., and a bending strength of greater than or equal to15 kgf/mm² after being heated to 570° C. for 2 hours. Glass I and glassII are desirable as the above-mentioned glasses. Glasses comprisingSiO₂, Al₂O₃, ZrO₂; a total of 3 to 15 mol % of Na₂O, and K₂O; a total of3 to 30 mol % of MgO, CaO, SrO, BaO, and ZnO (where the ratio of CaO tothe total content of MgO, CaO, SrO, and BaO is greater than or equal to0.5) are desirable. With the glasses of the above compositional ranges,glasses having the ratio of the BaO content to the total content of MgO,CaO, SrO, and BaO denoted as mol %, BaO/(MgO+CaO+SrO+BaO) is greaterthan or equal to 0.15 are desirable. With the above glasses, glassescomprising 47 to 70 mol % SiO₂ and 1 to 10 mol % Al₂O₃ (where the totalcontent of SiO₂ and Al₂O₃ is 57 to 80 mol %); and 1 to 12 mol % ZrO₂ arealso desirable. Glasses comprising SiO₂, Al₂O₃, CaO, BaO, Na₂O, K₂O, andZrO₂, as well as glasses comprising SiO₂, Al₂O₃, CaO, BaO, MgO, Na₂O,K₂O, and ZrO₂ are also desirable. In the same manner as substrate B,substrate A is desirably comprised of a glass exhibiting a bendingstrength of greater than or equal to 15 kgf/mm² after being heated to570° C. for two hours. Both substrates A and B preferably have a bendingstrength as set forth above of greater than or equal to 17 kgf/mm², morepreferably greater than or equal to 20 kgf/mm², and even more preferablygreater than or equal to 25 kgf/mm². When the above-described bendingstrength following heating is greater than or equal to 15 kgf/mm², asubstrate for information recording media that is capable of maintaininghigh strength after heat treatment can be obtained. Within theabove-stated ranges, it suffices to set the above bending strength toless than or equal to 100 kgf/mm² to obtain a glass of high stability.The substrate for information recording media of the present inventionhaving a bending strength within the above-stated range undergoes littlerelaxation following high-temperature heat treatment of the compressionstress layer near the glass surface formed by chemical reinforcement.Thus, based on this substrate, it is possible to provide a substrate forglass information recording media capable of maintaining high strengtheven after heat treatment. Due to this property, it is possible tomaintain a desired strength even when a film formed at high temperatureon a substrate comprised of glass having a bending strength within theabove-stated range is subjected to high-temperature treatment such ashigh-temperature annealing, and the strength of the substrate remainshigh even with exposure to sharp temperature change during theabove-mentioned high-temperature treatment. Thus, the substrate tendsnot to break and is easy to handle.

The bending strength can be measured in a three-point bending test overa 30 mm span with a rate of weight increase of 0.5 mm/s using a thinsheet sample measuring 40×10×1 mm. The edge surfaces (40×1 mm surfacesand 10×1 mm surfaces) of the sample employed are optically polished.

In the substrate for information recording media of the presentinvention, when the bending strength before chemical reinforcement ofthe glass constituting the substrate is denoted as f_(b) and the bendingstrength after maintaining the chemically reinforced glass at atemperature T [° C.] (where T denotes a temperature selected between 20and 570° C.) for two hours is denoted as f_(T), the value of(f_(T)−f_(b))/f_(b) is desirably greater than or equal to 0.5,preferably greater than or equal to 0.52. In the substrate forinformation recording media of the present invention, when the value of(f_(T)−f_(b))/f_(b) is greater than or equal to 0.5, it is possible toobtain a substrate for information recording media having adequatebending strength during the formation and heat treatment of theinformation recording layer; for example, the formation and heattreatment of an information recording layer on an information recordingmedium in a perpendicular magnetic recording system. The above-mentionedheating for two hours at temperature T is conducted in air. From theperspective of achieving better chemical reinforcement whilecontributing to higher stability of the glass, the value of(f_(T)−f_(b))/f_(b) is desirably kept less than or equal to 9. Thedetermination of whether or not the value of (f_(T)−f_(b))/f_(b) isgreater than or equal to a prescribed value is made by measuring thebending strength f_(b) and the bending strength f₅₇₀ of theabove-described glass after maintaining it for two hours at 570° C.following chemical reinforcement, calculating the value of(f₅₇₀−f_(b))/f_(b), and confirming that this value is greater than orequal to the desired value. The determination of whether or not thevalue of (f_(T)−f_(b))/f_(b) is less than or equal to a desired value ismade by measuring the bending strength f_(b) and the bending strengthf₅₇₀ of the above-described glass after maintaining it for two hours at570° C. following chemical reinforcement, calculating the value of(f₅₇₀−f_(b))/f_(b), and confirming that this value is less than or equalto the desired value.

In the substrate for information recording media of the presentinvention, the value of (f₂₀−f_(b))/f_(b) for a bending strength f₂₀ at20° C. is desirably greater than or equal to 1, preferably greater thanor equal to 1.2. From the perspectives of achieving better stability andchemical reinforcement of the glass, the value of (f₂₀−f_(b))/f_(b) isdesirably less than or equal to 9.

In the substrate for information recording media of the presentinvention, the average coefficient of linear thermal expansion at 30 to300° C. of the glass constituting the substrate is desirably greaterthan or equal to 60×10⁻⁷K⁻¹, preferably from 60×10⁻⁷ to 120×10⁻⁷K⁻¹, andmore preferably from 70×10⁻⁷ to 120×10⁻⁷K⁻¹. Imparting an averagecoefficient of linear thermal expansion within the above-stated range tothe substrate for information recording media of the present inventionyields a glass with thermal expansion characteristics close to those ofa metal. Thus, it is possible to provide a substrate for glassinformation recording media that can be firmly secured with a metalfixture. The material constituting this fixture can be a metal such asstainless steel, or a ceramic with closer thermal expansioncharacteristics.

For the same reason as above, at 100 to 300° C., an average coefficientof linear thermal expansion of greater than or equal to 70×10⁻⁷K⁻¹ isdesirable, from 70×10⁻⁷ to 120×10⁻⁷K⁻¹ is preferred, and from 75×10⁻⁷ to120×10⁻⁷K⁻¹ is of even greater preference.

The liquid phase temperature of the glass constituting the substrate forinformation recording media of the present invention is desirably lessthan or equal to 1,200° C., preferably 1,050° C. A liquid phasetemperature of less than or equal to 1,200° C. yields a glass havinggood resistance to devitrification.

[Manufacturing Methods]

The glass for chemical reinforcement and the glass constituting thesubstrate for information recording media of the present invention canbe manufactured by known manufacturing methods using existing equipment.For example, a homogeneous glass melt obtained by the high-temperaturemelt method—that is, melting prescribed proportions of glass startingmaterials in air or in an inert gas atmosphere, and homogenizing theglass by bubbling, stirring, or the like—can be molded into a desiredform such as sheet glass by a known molding method such as pressing,down drawing, or floating. However, in glass containing at a minimumSb₂O₃ or As₂O₃, since the Sb₂O₃ or As₂O₃ reacts with the molten metalemployed in the floating method, it is desirable to employ the pressingor down drawing method, with the pressing method being particularlydesirable. The reason the glass for chemical reinforcement and the glassconstituting the substrate for information recording media of thepresent invention can be molded by the usual methods despite having theabove-described high glass transition temperatures is that the liquidphase temperature of these glasses, at less than or equal to 1,200° C.,is low and they have good resistance to devitrification.

The chemical reinforcement treatment of the glass for chemicalreinforcement and of the substrate for information recording media ofthe present invention can be conducted by known methods such asimmersing the glass in molten salt. A molten salt comprising potassiumnitrate is desirably employed. Specifically, the molded glass isimmersed in the molten salt of an alkali metal, preferably a molten saltcontaining potassium (for example, potassium nitrate molten salt) toexchange alkali metal ions in the molten salt for alkali metal ions inthe glass (particularly those near the glass surface), thereby forming acompressive stress (chemically reinforced) layer on the glass surface.The ion exchange is desirably conducted until the above-describeddesirable properties are achieved. In most chemical reinforcement,lithium ions in the glass also exchange with sodium ions and/orpotassium ions in the molten salt. In that case, a large quantity ofLi₂O must be incorporated into the glass. When the quantity of Li₂Orequired by ion exchange is incorporated, the glass transitiontemperature drops sharply. Thus, in the present invention, it isdesirable to conduct chemical reinforcement by means of an ion exchangetreatment that replaces sodium ions with potassium ions. However, whenLi₂O is introduced into the glass, it is possible to conduct chemicalreinforcement by having lithium ions in the glass exchange with sodiumions in the molten salt and sodium ions in the glass exchange withpotassium ions in the molten salt.

Since the substrate for information recording media of the presentinvention affords high strength due to good chemical reinforcement, hasgood melt properties, and has a high glass transition temperature, iscan be suitably employed as a substrate for information recording mediain perpendicular magnetic recording systems.

The present invention can provide substrates for information recordingmedia such as the following:

-   (1) A substrate of glass having a specific gravity of from 2.4 to    3.0, preferably from 2.4 to 2.9.-   (2) A substrate of glass having a modulus of rigidity of greater    than or equal to 30 GPa, preferably from 30 to 35 GPa.-   (3) A substrate of glass having a specific modulus of greater than    or equal to 26×10⁶ Nm/kg, preferably from 26×10⁶ to 32×10⁶ Nm/kg.-   (4) A substrate of glass having a Poisson ratio of from 0.22 to    0.25.

The use of the substrate for an information recording medium of thepresent invention having these various properties makes it possible toprovide an information recording medium and a substrate for informationrecording media that are stable and can be rotated at high speed.

For example, when manufacturing a disk substrate, a molded glass membercan be processed to make it round, center punched, and the inner andouter perimeter surfaces thereof processed, ground, and polished toobtain a disk-shaped information recording medium substrate of desiredsize. During polishing, lapping can be conducted with a polishingmaterial or diamond pellets and polishing can be conducted with apolishing material such as cerium oxide to achieve a surface precisionfalling within the range of, for example, 0.1 to 0.6 nm. Followingprocessing, the substrate surface is desirably washed with a cleaningsolution to clean it. Next, the substrate is immersed in a molten saltcontaining potassium nitrate at a prescribed temperature, chemicallyreinforced, and washed again to obtain a clean substrate. An alkali oracid solution such as hydrofluorosilicic acid solution, or an organicsolvent can be suitably selected for use as the cleaning solution.

The substrate for information recording media of the present inventioncan be employed as a substrate for magnetic recording media. Inparticular, it can be suitably employed as a substrate for magneticrecording media in perpendicular magnetic recording systems. That is,since the substrate for information recording media of the presentinvention has a glass transition temperature that is substantiallyhigher than the temperature reached during heat treatment and anadequately high Young's modulus, it yields a substrate that does notdeform during heat treatment during the manufacturing process and doesnot elastically deform during high-speed rotation.

[Information Recording Media]

The information recording medium of the present invention can bemanufactured by providing an information recording layer on theabove-described substrate for information recording media. Since thechemically reinforced glass having good thermal resistance and highstrength of the present invention is employed as substrate, theinformation recording medium of the present invention affords theadvantages of high strength and permitting high-temperature processing.By suitably selecting the information recording layer, it is possible toemploy the above-described information recording medium in various formsof information recording media. Examples of such media are magneticrecording media, photomagnetic recording media, and optical recordingmedia.

Since the information recording medium of the present invention has bothhigh thermal resistance and high strength, it is particularly suited touse as a magnetic recording medium in perpendicular magnetic recordingsystems. Information recording media employed in perpendicular magneticrecording systems can provide information recording media capable ofhandling higher recording densities. That is, the magnetic recordingmedia employed in perpendicular magnetic recording systems have arecording density (for example, 1 TBit/(2.5 cm)²) that is higher thanthe surface recording density (100 GBit/(2.5 cm)² or more) of theconventional magnetic recording media employed in longitudinal magneticrecording systems. Thus, higher density recording can be achieved.

The information recording medium of the present invention and the methodof manufacturing the same will be specifically described below.

In the above information recording medium, an information recordinglayer is present on the above-described substrate for informationrecording media. It is possible to manufacture an information recordingmedium such as a magnetic disk by sequentially depositing on theabove-described glass substrate, an underlayer, a magnetic layer, aprotective layer, a lubricating layer, and the like. The magnetic layer(information recording layer) is not specifically limited; by way ofexample, it may be in the form of a Co—Cr based (here the term “based”means a material comprising the stated substances), Co—Cr—Pt based,Co—Ni—Cr based, Co—Ni—Pt based, Co—Ni—Cr—Pt based, or Co—Cr—Ta basedmagnetic layer. A Ni layer, Ni—P layer, Cr layer, or the like may beemployed as the underlayer. Examples of the materials employed inmagnetic layers (information recording layer) suited to high recordingdensity are CoCrPt-based alloy materials and, above all, CoCrPtB-basedalloy materials. FePt-based alloy materials are also suitable. Thesemagnetic layers are particularly useful when employed as magneticmaterials in perpendicular magnetic recording systems. CoCrPt-basedmaterials can be used to form films at elevated temperatures of from 300to 500° C. and FePt-based alloy materials can be used to form films atelevated temperatures of from 500 to 600° C., or heat treated followingfilm formation, to adjust the crystal orientation or crystal structureand achieve a structure suited to high recording density.

A nonmagnetic underlayer and/or soft magnetic underlayer can be employedas the underlayer. A nonmagnetic underlayer is chiefly provided toreduce the size of the crystal grains of the magnetic layer or controlthe crystal orientation of the magnetic layer. A bcc-based crystalunderlayer such as a Cr-based underlayer has the effect of promotingin-plane orientation, and is thus desirably employed in magnetic disksemployed in in-plane (longitudinal) recording systems. hcp-basedcrystalline underlayers such as Ti-based underlayers and Ru-basedunderlayers have the effect of promoting vertical orientation, and maythus be employed in magnetic disks employed in perpendicular magneticrecording systems. Amorphous underlayers have the effect of reducing thesize of the crystal grains in the magnetic layer.

Soft magnetic underlayers are primarily employed in perpendicularmagnetic recording disks to enhance magnetic pattern recording onperpendicular magnetic recording layers (magnetic layers) by magneticheads. Fully utilizing the effect of a soft underlayer requires a layerwith a high saturation magnetic flux density and high magneticpermeability. Thus, it is desirable to conduct film formation at hightemperature or conduct a heat treatment following film formation.Examples of such soft magnetic layer materials are Fe-based softmagnetic materials such as FeTa-based soft magnetic materials andFeTaC-based soft magnetic materials. CoZr-based soft magnetic materialsand CoTaZr-based soft magnetic materials are also desirable.

A carbon film or the like can be employed as the protective layer. Aperfluoropolyether-based lubricant or the like can be employed to formthe lubricating layer.

An example of a desirable mode of a perpendicular magnetic recordingdisk is a magnetic disk obtained by sequentially forming on the glasssubstrate of the present invention a soft magnetic underlayer, amorphousnonmagnetic underlayer, crystalline nonmagnetic underlayer,perpendicular magnetic recording layer (magnetic layer), protectivelayer, and lubricating layer.

In the case of a magnetic recording medium employed in a perpendicularmagnetic recording system, the configuration of the films formed on thesubstrate may be in the form of a single-layer film consisting of aperpendicular magnetic recording layer formed on a glass substrate ofnonmagnetic material, a two-layer film consisting of a soft magneticlayer and a magnetic recording layer sequentially deposited on the glasssubstrate, and a three-layer film consisting of a hard magnetic layer,soft magnetic layer, and magnetic recording layer sequentially depositedon the glass substrate. Of these, the two-layer film and three-layerfilm are preferred because they are better suited than the single-layerfilm to high recording density and maintaining a stable magnetic moment.

The characteristics of such multilayer magnetic film perpendicularmagnetic recording media can generally be enhanced by high-temperatureheat treatment (annealing) during or after film formation in ahigh-temperature sputtering unit at 300 to 600° C., preferably 400 to600° C., to expose the substrate to an elevated temperature of 300 to600° C., preferably 400 to 600° C. Since the substrate for informationrecording media of the present invention is comprised of glass having aglass transition temperature (Tg) of greater than or equal to 620° C.,good smoothness can be maintained without deformation of the substratein the above high-temperature heat treatment. Accordingly, the presentinvention yields an information recording medium such as a magnetic diskequipped with the above-described films on a flat substrate. Further,the above-described high-temperature heat treatment is conducted afterchemical reinforcement of the substrate, but in the informationrecording medium of the present invention, since there is littlerelaxation of the compression stress layer in the substrate surface evenafter heat treatment, it is possible to obtain an information recordingmedium such as a magnetic disk of adequate mechanical strength. Thedimensions of the substrate (such as a magnetic disk substrate) forinformation recording media and the information recording medium (suchas a magnetic disk) of the present invention are not specificallylimited. However, it is possible to reduce the size of the medium andthe substrate to achieve a high recording density. Thus, the presentinvention can be applied not just to standard 2.5 inch diameter, butalso smaller diameter (such as 1 inch) magnetic disk substrates andmagnetic disks.

EMBODIMENTS

The present invention will be described next in greater detail throughembodiments. The present invention is not limited to these embodiments.

Embodiments 1 to 9

Denoted as mol %, starting materials in the form of SiO₂, Al₂O₃,Al(OH)₃, CaCO₃, BaCO₃, Na₂CO₃, K₂CO₃, TiO₂, and ZrO₂ were weighed out toobtain from 300 to 1,500 g of glass starting materials designed to yieldglasses having the compositions shown in Tables 1 and 2. These wereintimately mixed to prepare batches, and the batches were charged toplatinum crucibles and melted for 3 to 8 hours in air at a temperatureof from 1,400 to 1,600° C. After melting, the glass melts were pouredinto carbon molds measuring 40×40×20 mm, cooled to the glass transitiontemperature, immediately placed in an annealing furnace, maintained forone hour, and then left to cool to room temperature in the furnace. Whenthe glasses obtained were observed by microscope, no crystal grains werefound. The glasses obtained had a high degree of homogeneity, exhibitedno unmelted material, and were confirmed to have good melt properties.The glasses obtained in this manner were processed into thin sheetsmeasuring 40×10×1 mm from which samples of glass for chemicalreinforcement were prepared. The glasses were then processed into disksubstrates with an outer diameter of 65.0 mm, a center hole diameter of20.0 mm, and a thickness of 0.635 mm. The glass samples for chemicalreinforcement and the disk substrates were processed by polishing theirprincipal surfaces to render them flat and smooth. Surfaces other thanthe principal surfaces were also polished to eliminate minute scratchesthat could decrease strength, yielding smooth surfaces. Chemicalreinforcement was conducted by immersing the disk substrates for theperiods stated in Tables 1 and 2 in potassium nitrate molten salt at thetemperatures stated in Tables 1 and 2.

Single samples of glass for chemical reinforcement were paired withsingle disk substrates to obtain nine pairs, or a total of 18 samples.The glass transition temperature, sag temperature, average coefficientof linear expansion at 30 to 300° C., average coefficient of linearexpansion at 100 to 300° C., specific gravity, Young's modulus, modulusof rigidity, Poisson ratio, specific modulus, liquid phase temperature,bending strength f_(b) prior to chemical reinforcement, bending strengthf₂₀ after being maintained for 2 hours at 20° C. following chemicalreinforcement, and bending strength f₅₇₀ after being heated in air for 2hours at 570° C. after chemical reinforcement of each sample were thenmeasured. The chemical reinforcement conditions, characteristics, andglass composition of each pair are given in Tables 1 and 2. A glass wasprepared from each of the pairs by adding 0.5 weight % of Sb₂O₃ as anouter percentage and the same characteristics were measured. In theglasses to which Sb₂O₃ was added, microscopic examination revealedabsolutely no bubbles.

The methods of measuring the various characteristics are indicatedbelow.

-   (1) Glass transition temperature and sag temperature

Glass identical to the above-described samples was processed into ashape 5 mm in diameter×20 mm. A thermomechanical analyzer (TMA 8140)made by Rigaku Corp. was employed to conduct measurement at a rate oftemperature increase of +4° C./min. SiO₂ was employed as the referencesample. The glass transition temperature was the temperature at whichthe viscosity of the glass reached 10^(13.3) dPa·s.

-   (2) Average coefficient of linear thermal expansion

Measured simultaneously with the glass transition temperature.

-   (3) Specific gravity

Glass identical to the above-described samples was processed into ashape measuring 40×20×15 mm and measurement was conducted by Archimedes'method.

-   (4) Young's modulus, modulus of rigidity, Poisson ratio

Glass identical to the above-described samples was processed into ashape measuring 40×20×15 mm and measurement was conducted by anultrasonic method.

-   (5) Specific modulus

Calculated from the above Young's modulus and specific gravity by theequation (specific modulus=Young's modulus/specific gravity).

-   (6) Liquid phase temperature

The glass samples were placed in a platinum container with lid, fullymelted at 1,500° C., maintained in a furnace set to a prescribedtemperature, removed after a prescribed period had elapsed, and observedby optical microscopy to determine whether crystals had formed in theglass. The lowest temperature at which crystals were not produced wasadopted as the liquid phase temperature.

-   (7) Bending strength

Measurement was conducted on thin-sheet samples (40×10×1 mm, lateralsurfaces polished). Specifically, the three-point bending strength wasmeasured over a 30 mm span at a weight increase rate of 0.5 mm/s.

As will be clear from Tables 1 and 2, the glass for chemicalreinforcement and disk substrates of the present embodiments exhibitedgood characteristics in the form of a glass transition temperature ofgreater than 620° C., an average coefficient of linear thermal expansionat 30 to 300° C. of greater than or equal to 60×10⁻⁷K⁻¹, an averagecoefficient of linear thermal expansion at 100 to 300° C. of greaterthan or equal to 70×10⁻⁷K⁻¹, a specific gravity of from 2.4 to 3.0, aYoung's modulus of greater than or equal to 75 GPa, a modulus ofrigidity of greater than or equal to 30 GPa, a specific modulus ofgreater than or equal to 26×10⁶ Nm/kg, a Poisson ratio of from 0.22 to0.25, a bending strength following chemical reinforcement of greaterthan or equal to 15 kgf/mm², a bending strength following two hours ofheating at 570° C. of greater than or equal to 15 kgf/mm², an(f_(T)−f_(b))/f_(b) of greater than or equal to 0.5, and an(f₂₀−f_(b))/f_(b) of greater than or equal to 1.

Each of the disk-shaped glass substrates of the present embodiment wassuitable for use as a substrate in a standard 2.5-inch informationrecording medium. In particular, as substrates with high resistance toheat and high strength, they were suited for use as substrates inmagnetic recording media, particularly magnetic recording media employedin perpendicular magnetic recording systems.

Embodiment 10

Disk-shaped substrates with an outer diameter of 27.4 mm, a center holediameter of 7.0 mm, and a thickness of 0.381 mm were prepared fromglasses having compositions identical to the glasses of Embodiments 1 to9 and the glasses of Embodiment 1 to 9 to which Sb₂O₃ had been added.Specifically, homogenized glass melts were fed into a pressing mold,press molded, and gradually cooled to obtain disk-shaped substrates.These were subjected to mechanical processing such as grinding andpolishing and then chemically reinforced. In addition to press molding,examples of methods that can be used to prepare the above-describedsubstrate include the use of a method known such as float molding toform thin sheets of glass and processing these thin sheets of glass intoa disk shape. The substrates for information recording media thusobtained were washed with a cleaning solution. These substrates werefound to have good characteristics in the form of a bending strengthafter chemical reinforcement of greater than or equal to 15 kgf/mm², abending strength after heating for 2 hours at 570° C. of greater than orequal to 15 kgf/mm², an (f_(T)−f_(b))/f_(b) value of greater than 0.5,and an (f₂₀−f_(b))/f_(b) value of greater than or equal to 1.

These substrates were suitable for use as substrates in standardone-inch information recording media, and in particular, as substrateswith high thermal resistance and strength, such as substrates employedin magnetic recording media, particularly the substrates in magneticrecording media employed in perpendicular magnetic recording systems.These substrates were washed with a cleaning solution. Since the amountof alkali dissolving out of the glass constituting the substrates wasextremely low, it was possible to prevent roughening of the surface ofthe substrate during cleaning. The center line average roughness Ra ofthe principal surface of each of the glass substrates following cleaningwas from 0.1 to 0.6 nm.

The center line average roughness Ra of the glass substrates wasmeasured by atomic force microscopy (AFM).

Embodiment 11

Magnetic disks for use in a perpendicular magnetic recording system wereprepared from the glass substrates of Embodiment 10 following drying. Inthe formation of the magnetic recording layer, a two-layer film in whicha soft magnetic layer and a magnetic recording layer were sequentiallydeposited, and a three-layer film, in which a hard magnetic layer, softmagnetic layer, and magnetic recording layer were sequentiallydeposited, were employed to manufacture two types of magnetic disks foruse in a perpendicular magnetic recording system. In this process, themagnetic recording film was subjected to a high-temperature heattreatment at 400 to 600° C. However, since each of the substrates had athermal resistance in the form of a glass transition temperature (Tg) ofgreater than or equal to 620° C., the substrates did not deform and ahigh degree of flatness was maintained. The various magnetic disksdescribed above were then subsequently manufactured in the same manneras set forth above.

Due to the high glass transition temperature of the glass substrates ofthe present invention, they were suited to high-temperature processingto enhance the characteristics of the magnetic recording medium and tothe preparation of magnetic films in a high-temperature sputteringdevice.

Although the example of a magnetic recording medium is described in theabove embodiments, good results can be similarly achieved withsubstrates employed in other information recording media and with otherinformation recording media, such as substrates and media employed inoptical recording systems and photomagnetic recording systems. TABLE 1Embod. 1 Embod. 2 Embod. 3 Embod. 4 Embod. 5 Glass SiO₂ 63.0 63.0 63.063.0 63.0 composition Al₂O₃ 4.0 4.0 4.0 4.0 4.0 (mol %) CaO 13.0 13.013.0 13.0 13.0 BaO 3.0 3.0 3.0 3.0 3.0 CaO + BaO 16.0 16.0 16.0 16.016.0 Na₂O 4.0 4.0 4.0 4.0 4.0 K₂O 5.0 5.0 5.0 5.0 5.0 Na₂O + K₂O 9.0 9.09.0 9.0 9.0 TiO₂ 4.0 4.0 4.0 4.0 4.0 ZrO₂ 4.0 4.0 4.0 4.0 4.0 Total 100100 100 100 100 Glass transition temperature [° C.] 679 679 679 679 679Sag temperature [° C.] 756 756 756 756 756 Average coefficient of linear79.5 79.5 79.5 79.5 79.5 expansion at 30-300° C. [×10⁻⁷ K⁻¹] Averagecoefficient of linear 83.3 83.3 83.3 83.3 83.3 expansion at 100-300° C.[×10⁻⁷ K⁻¹] Specific gravity 2.79 2.79 2.79 2.79 2.79 Young's modulus[GPa] 82.7 82.7 82.7 82.7 82.7 Modulus of rigidity [GPa] 33.4 33.4 33.433.4 33.4 Poisson ratio 0.24 0.24 0.24 0.24 0.24 Specific modulus [×10⁶Nm/kg] 29.6 29.6 29.6 29.6 29.6 Liquid phase temperature [° C.] 1050 orless 1050 or less 1050 or less 1050 or less 1050 or less Ion exchangetemperature [° C.] 400 420 450 470 500 Ion exchange time [hours] 3 3 3 33 Bending Before chemical 12.0 12.0 12.0 12.0 12.0 strengthreinforcement f_(b) [kgf/mm²] After chemical 28.4 30.7 37.2 40.9 45.3reinforcement f₂₀ After two hours 18.6 22.3 24.5 25.9 31.1 heating at570° C. f₅₇₀ (f₅₇₀ − f_(b))/f_(b) 0.550 0.858 1.04 1.16 1.59 (f₂₀ −f_(b))/f_(b) 1.37 1.56 2.10 2.41 2.78

TABLE 2 Embod. 6 Embod. 7 Embod. 8 Embod. 9 Glass SiO₂ 63 63 65 65composition Al₂O₃ 4 5 5 5 (mol %) CaO 13 12 13 12 BaO 3 3 3 4 CaO + BaO16 15 16 16 Na₂O 5 5 4 4 K₂O 5 5 6 6 Na₂O + K₂O 10 10 10 10 TiO₂ 3 3 0 0ZrO₂ 4 4 4 4 Total 100 100 100 100 Glass transition temperature [° C.]663 670 671 671 Sag temperature [° C.] 744 747 749 753 Averagecoefficient of linear 84 83 79 78 expansion at 30-300° C. [×10⁻⁷ K⁻¹]Average coefficient of linear 87 86 83 82 expansion at 100-300° C.[×10⁻⁷ K⁻¹] Specific gravity 2.78 2.78 2.76 2.77 Young's modulus [GPa]82.3 82.3 81.5 79.9 Modulus of rigidity [GPa] 33.2 82.2 33 32.3 Poissonratio 0.238 0.237 0.24 0.24 Specific modulus [×10⁶ Nm/kg] 29.6 29.6 29.628.8 Liquid phase temperature [° C.] 1050 or less 1050 or less 1050 orless 1050 or less Ion exchange temperature [° C.] 420 420 420 420 Ionexchange time [hours] 3 3 3 3 Bending Before chemical 11.0 11.0 12.013.0 strength reinforcement f_(b) [kgf/mm²] After chemical 32.0 33.229.5 30.1 reinforcement f₂₀ After two hours 16.5 19.3 20.1 21.0 heatingat 570° C. f₅₇₀ (f₅₇₀ − f_(b))/f_(b) 0.50 0.75 0.67 0.62 (f₂₀ −f_(b))/f_(b) 1.9 2.02 1.46 1.32

INDUSTRIAL APPLICABILITY

The chemically reinforced glass of the present invention is useful as asubstrate for information recording media. In particular, it is usefulas a substrate for magnetic recording media suited to the manufacture inhigh-temperature sputtering devices of magnetic films employed inperpendicular magnetic recording systems. Further, the substrate forinformation recording media employing the chemically reinforced glass ofthe present invention can be employed in magnetic recording media andthe like.

1. A glass for use in chemical reinforcement for use in a substrate ofan information recording medium, having a composition comprising,denoted as mol %: Si0₂ 47 to 70%  Al₂0₃ 1 to 10% (where the total ofSi0₂ and Al₂0₃ is 57 to 80%) CaO 2 to 25% BaO 1 to 15% Na₂0 0 to 15% K₂01 to 10% (where the total of Na₂0 and K₂0 is 3 to 16%) Zr0₂ 1 to 12% MgO0 to 10% SrO 0 to 15% (where the ratio of the content of CaO to thetotal of MgO, CaO, SrO, and BaO is greater than or equal to 0.5) ZnO 0to 10% (where the total of MgO, CaO, SrO, BaO, and ZnO is 3 to 30%) Ti0₂0 to 10% and the total content of the above-stated components is greaterthan or equal to 95%.


2. The glass for use in chemical reinforcement of claim 1 characterizedin that the ratio of the BaO content to the total content of MgO, CaO,SrO, and BaO is greater than or equal to 0.15.
 3. A glass for use inchemical reinforcement for use in the substrate of an informationrecording medium employed in a perpendicular magnetic recording system,in which the glass exhibits the glass transition temperature is greaterthan or equal to 600° C.
 4. The glass for use in chemical reinforcementof any of claims 1 to 3 which has a Young's modulus of greater than orequal to 75 GPa.
 5. A substrate for use in an information recordingmedium characterized by consisting of the glasses of claim 4 and beingchemically reinforced.
 6. The substrate for use in an informationrecording medium of claim 5 which employs a chemically reinforced glassin which the bending strength following heating for two hours at 570° C.to is greater than or equal to 15 kgf/mm².
 7. A substrate for aninformation recording medium characterized by consisting of a chemicallyreinforced glass having a glass transition temperature of greater thanor equal to 600° C. and exhibiting a bending strength following heatingfor two hours at 570° C. of greater than or equal to 15 kgf/mm².
 8. Thesubstrate for an information recording medium of claim 5 in which, whenthe bending strength of the glass constituting the substrate prior tochemical reinforcement is denoted as f_(b) and the bending strength ofthe glass when maintained for two hours at a temperature T [° C.] (whereT denotes any temperature of from 20 to 570° C.) after having beenchemically reinforced is denoted as f_(T), the value of(f_(T)−f_(b))/f_(b) is greater than or equal to 0.5.
 9. The substratefor use in an information recording medium of claim 8, wherein the valueof (f₂₀−f_(b))/f_(b) for the bending strength f₂₀ at T=20° C. is greaterthan or equal to
 1. 10. The substrate for use in an informationrecording medium of claim 5, wherein the average coefficient of linearexpansion at 30 to 300° C. of the glass constituting the substrate isgreater than or equal to 60×10⁻⁷K⁻¹.
 11. The substrate for use in aninformation recording medium of claim 5 that is chemically reinforced byan ion exchange treatment in which sodium ions are replaced withpotassium ions.
 12. The substrate for use in an information recordingmedium of claim 5 that is employed as a substrate for an informationrecording medium employed in a perpendicular magnetic recording system.13. An information recording medium characterized by comprising aninformation recording layer on the substrate for an informationrecording medium of claim
 5. 14. The information recording medium ofclaim 13 that is a magnetic recording medium employed in a perpendicularmagnetic recording system.
 15. The information recording medium of claim13 characterized by being manufactured by subjecting a substrate havingan information recording layer to a heat treatment at a maximumtemperature of 300 to 600° C.
 16. A method of manufacturing aninformation recording medium comprising a step of forming a multilayeredfilm comprising an information recording layer on the substrate for aninformation recording medium of claim 5, characterized by furthercomprising the heating of the substrate on which the multilayered filmhas been formed to a temperature of from 300 to 600° C.
 17. Thesubstrate for an information recording medium of claim 7 in which, whenthe bending strength of the glass constituting the substrate prior tochemical reinforcement is denoted as f_(b) and the bending strength ofthe glass when maintained for two hours at a temperature T [° C.] (whereT denotes any temperature of from 20 to 570° C.) after having beenchemically reinforced is denoted as f_(T), the value of(f_(T)−f_(b))/f_(b) is greater than or equal to 0.5.
 18. The substratefor use in an information recording medium of claim 7, wherein theaverage coefficient of linear expansion at 30 to 300° C. of the glassconstituting the substrate is greater than or equal to 60×10⁻⁷K⁻¹. 19.The substrate for use in an information recording medium of claim 7 thatis chemically reinforced by an ion exchange treatment in which sodiumions are replaced with potassium ions.
 20. The substrate for use in aninformation recording medium of claim 7 that is employed as a substratefor an information recording medium employed in a perpendicular magneticrecording system.
 21. An information recording medium characterized bycomprising an information recording layer on the substrate for aninformation recording medium of claim
 7. 22. The information recordingmedium of claim 14 characterized by being manufactured by subjecting asubstrate having an information recording layer to a heat treatment at amaximum temperature of 300 to 600° C.
 23. A method of manufacturing aninformation recording medium comprising a step of forming a multilayeredfilm comprising an information recording layer on the substrate for aninformation recording medium of claim 7, characterized by furthercomprising the heating of the substrate on which the multilayered filmhas been formed to a temperature of from 300 to 600° C.